This invention relates generally to methods and apparatus for reconstruction of volumetric computed tomographic (CT) images, and more particularly to methods and apparatus for extending exact helical cone beam reconstruction algorithm for volumetric CT to improve noise characteristics and dose efficiency.
In at least one known multi-detector row CT imaging systems, two-dimensional (2D) algorithms have been used to reconstruct tomographic images based on an approximation of cone beam (CB) geometry into fan beam (FB) geometry. As CB volumetric CT (VCT) technology becomes more prevalent, maintaining reconstruction accuracy has become more challenging. As a result of a significantly larger cone angle, CB-to-FB geometry approximations result in significant artifacts. To combat these artifacts, three dimensional (3D) reconstruction algorithms can be used in CB VCT. One such algorithm has been proposed by A. Katsevich in “Analysis of an exact inversion algorithm for spiral cone-beam CT,” Physics in Medicine & Biology vol. 47, pp. 2583-2598, 2002. The Katsevich algorithm has drawn extensive attention, because its shift-invariant filtered back projection structure is compatible with that employed by currently commercial CT scanners.
However, like some other exact CB reconstruction algorithms, the Katsevich algorithm may not make use of projection data very efficiently. To maintain reconstruction accuracy, it uses the Tam-window to handle data redundancy by discarding all redundant projection data. In fact, the projection data needed by the Katsevich algorithm for filtering occupy a detector area larger than the Tam-window, but only the filtered data within the Tam-window contribute to CB reconstruction. This means that, the projection data filtered by the Katsevich algorithm but outside the Tam-window are wasted, resulting in degraded noise characteristics and x-ray dose efficiency. The amount of wasted projection data is helical pitch-dependent. The larger the helical pitch is, the more the projection data are wasted.